In the manufacture of MEMS elements having mechanical element parts such as sensors and actuators integrated on substrates, the predominant portion is processed using the semiconductor integrated circuit fabrication technology. For forming movable structure parts and stereo-structure parts, the MEMS technology utilizes inherent processing techniques such as sacrificial layer etching technique, deep drilling/etching technique, and anisotropic etching technique. Among these techniques, the sacrificial layer etching technique is an important technique involving depositing a plurality of layers on a substrate, and selectively removing the lower layer called sacrificial layer, leaving the upper layer. With respect to the materials used in the sacrificial layer, many reports have been made. For example, JP-A 2000-255072 discloses a positive novolac resist composition which can be advantageously processed to form fine size features, but has poor heat resistance, by which a choice of the material of the upper layer is limited.
As a solution to this problem, one approach proposed thus far is by providing a positive resist composition containing a cresol novolac resin and adding a crosslinker thereto, followed by certain steps. Although this approach enables relatively fine size processing and has sufficient heat resistance, it has poor sensitivity and fails to reach a target resolution estimated in the future. While a chemically amplified positive photoresist composition is utilized mainly as the photoresist using excimer laser as the exposure light source, its sensitivity is controllable by adjusting the amount of a photoacid generator (PAG) added. Thus the composition can be easily tailored so as to have high sensitivity, as compared with a photoresist composition comprising a cresol novolac resin and naphthoquinonediazide. However, when the chemically amplified positive photoresist composition is used as a sacrificial film, the pattern formed via exposure and development will flow at the elevated temperature during film deposition thereon. No reports have been made as to the application of the chemically amplified positive photoresist composition to sacrificial film etching.
As the size of substrates increases (wafers of 300 mm or greater), a higher sensitivity is required for the purpose of improving throughputs. Particularly when large size glass substrates for liquid crystal displays are processed, the resist material is required to have a sensitivity of up to 250 mJ/cm2, especially up to 100 mJ/cm2.